The removal of air borne contaminants from the air is a concern to everyone. Gas phase filtration has traditionally been accomplished by methods which utilize activated carbon. One approach has been to use a carbon/adhesive slurry to glue the carbon to a substrate. However, the adhesive decreases carbon performance by forming a film on its surface. A second approach involves carbonizing an organic based web by heating, followed by carbon activation. This material has a high cost and has relatively low adsorption capacity. A third approach involves forming a slurry of carbon powders and fibers into sheets by a process analogous to a wet papermaking process. This material is medium-to-high cost, and has an undesirable high pressure drop.
Alternatively, carbon particles have been treated with chemicals to increase uptake of air contaminants. However, chemical treatment is not efficient when used in conjunction with an aqueous process, as the aqueous nature of the process either washes away the chemical used to impregnate the carbon, or reacts undesirably with the impregnating chemical rendering it useless. However, filter materials which do not incorporate chemical absorbents into the carbon particles perform far less effectively than those which do include chemically impregnated absorbents.
Another approach to entrain air contamination has been to produce low, medium and high efficiency pleatable composite filter media which include either a low, medium or high efficiency fibrous filtration layer of randomly oriented fibers; and one or more permeable stiffening layers which enable the composite filter media to be pleated and to sustain its shape. Such filtration devices serve as vehicle passenger compartment air filters, high performance engine air filters and engine oil filters. ASHRAE (American Society of Heating Refrigeration and Air Conditioning Engineers) pleatable filters and the like typically use a pleated high efficiency filtration media for the filtration element.
Currently, the pleated high efficiency media normally used in these filtration devices are made from ASHRAE filter media or paper products. These paper products are made by a wet-laid technique wherein fibers, e.g. glass or cellulosic fibers, are dispersed in an aqueous binder slurry which is stirred to cause the fibers to become thoroughly and randomly mixed with each other. The fibers are then deposited from the aqueous binder slurry onto a conventional paper making screen or wire as in a Fourdrinier machine or a Rotoformer machine to form a matted paper which includes a binder resin, e.g., a phenolic resin. Pleated filter elements made from such papers can exhibit high efficiencies. However, these pleated filter elements have low dirt-holding capacities and exhibit high pressure drops.
Electrostatically charged synthetic filter media is also used in these filtering applications, and these can attain very high filtration versus pressure drop performance characteristics, at least in their initial charge state. However, during use many of these products lose their electrostatic charge, or it is masked by deposits, causing filtration efficiency to drop substantially, sometimes to levels below what is acceptable.
Accordingly, there remains a need to provide a relatively low cost, high efficiency filter media for these filtration applications which exhibit relatively high dirt-holding and/or air contaminant capacities and relatively low pressure drops as well as low and medium efficiency filter media which exhibit relatively high dirt-holding capacities and relatively low pressure drops.